Update free-courses-en.md

[Divyanshu9794]

What does this PR do?

Add resource(s) | Remove resource(s) | Add info | Improve repo

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For resources

Description

Why is this valuable (or not)?

How do we know it's really free?

For book lists, is it a book? For course lists, is it a course? etc.

Checklist:

• Search for duplicates.
• Include author(s) and platform where appropriate.
• Put lists in alphabetical order, correct spacing.
• Add needed indications (PDF, access notes, under construction).
• Used an informative name for this pull request.

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!

[eshellman]

For youtube, we prefer "playlist" links and the name of the producer/creator. docs/CONTRIBUTING.md#platforms-and-access-notes